Diabetes mellitus (DM) is a disease in which a blood sugar level (glucose concentration in the blood) is pathologically elevated by insulin secretion failure or insulin resistance, and is known to be a risk factor for various severe complications. It is said that diabetes mellitus is developed by various environmental factors (such as lack of exercise, overeating, and obesity) involved, based on genetic factors, and it is expected that in the future, the number of diabetic patients will increase as the number of obese people increases. Diabetes mellitus is classified into an insulin-dependent type 1 diabetes mellitus (IDDM: Insulin Dependent Diabetes Mellitus) and non-insulin-dependent type 2 diabetes mellitus, and a majority (about 90%) of patients with diabetes mellitus are classified into the type 2 diabetes mellitus.
The type 1 diabetes mellitus is a disease in which β cells that secrete insulin in the islets of Langerhans in the pancreas die by various genetic factors or acquired factors, and the type 2 diabetes mellitus is a disease which is caused by reduction of the amount of insulin secreted in response to glucose in β cells and a decrease in insulin sensitivity in the peripheral tissues (such as the liver, the muscle, and fat).
With regard to treatment and prevention related to diabetes mellitus, drug therapy has also been performed, in addition to diets and exercise therapy.
Examples of the typical drug therapy at present include drug therapies in which insulin, insulin analogues, GLP-1 (glucagon-like peptide-1) analogues, or the like is subcutaneously administered, and drug therapies in which a hypoglycemic agent that can be orally administered is used Examples of the hypoglycemic agent that can be orally administered include sulfonylurea agents (SU drugs) such as glimepiride; biguanide agents (BG drugs) such as metformin; α-glucosidase inhibitors (αGI drugs) such as voglibose and miglitol; thiazolidine-based derivatives (TZD drugs) such as pioglitazone; and DPP-IV (dipeptidyl peptidase IV) inhibitors such as sitagliptin and alogliptin.
Somatostatin is widely distributed in the central nervous system, such as hypothalamus, islets of Langerhans in pancreas, intestinal mucosa, or the like, and plays an important role in the gastrointestinal motility, secretion of digestive juice, and regulation of glucose or lipid metabolism. In particular, in vivo, it has been known that somatostatin performs an inhibitory action on production or secretion of various hormones, proliferation factors, and physiologically active substances. Examples of the hormone on which somatostatin performs an inhibitory action include growth hormone (GH), thyroid stimulating hormone (TSH), prolactin, insulin, glucagon, gastrin, secretin, PYY (peptide YY), GIP (gastric inhibition polypeptide), GLP-1, CCK (cholecystokinin), VIP (vasoactive intestinal peptide), and Oxyntomodulin. Further, somatostatin also acts as paracrine in the islets of Langerhans in the pancreas or the mucosa of the digestive tract, in which δ cells are in contact with α cells and β cells. For this reason, somatostatin has various physiological functions in the endocrine system, the exocrine system, the nervous system, or the like.
The somatostatin receptor is one of the seven-transmembrane G protein-coupled receptors subtypes, and 5 kinds of subtypes thereof have now been found, which are named SSTR1, SSTR2, SSTR3, SSTR4, and SSTR5, respectively (Non-Patent Literature 1). Among these, SSTR5 has been shown to be involved in the regulation of secretion of insulin and incretins (Non-Patent Literature 2).
On the other hand, in Patent Literature 1, it has been reported that the following compound has an SSTR5 antagonistic action.

[in which Ra is independently selected from the group consisting of a hydrogen atom, a halogen atom, a C1-10 alkyl group, and a C1-10 alkyl group substituted with a halogen atom,
R1 is selected from the group consisting of a hydrogen atom, a substituted phenyl, and a substituted heterocycle,
R2 is selected from the group consisting of a substituted aryl and a substituted heterocycle, and
n and m are independently selected from the group consisting of 1, 2, and 3.]
Furthermore, in Patent Literature 2, it has been reported that the following compound has an SSTR5 antagonistic action.

[in which Ra is independently selected from the group consisting of a hydrogen atom, a halogen atom, a C1-10 alkyl group, and a C1-10 alkyl group substituted with a halogen atom,
R1 is selected from the group consisting of a substituted phenyl and a substituted heterocycle, and
R2 is selected from the group consisting of a substituted phenyl group and a substituted heterocycle.]